1. Field of the Invention
This invention pertains in general to power supplies for protective relays and more particularly to power supplies that include a backup power source in the event of a line voltage collapse.
2. Background Information
Protective relays are installed in electric power transmission and distribution facilities to detect overloads, short circuits, and other fault conditions. They are connected to power circuit breakers or switching devices that disconnect the faulty portion of the network on command from the relay, to isolate the problem. Reliable operation of relays is especially critical during short circuit faults, which must be rapidly isolated to minimize damage to equipment, plus the risk of fire and injury of personnel.
Overcurrent protective relays typically monitor the current load within a circuit downstream of a circuit breaker through current transformers which communicate with each phase of the monitored circuit. The relay monitors the time-current characteristics of the load through which it senses short circuit faults or overloads on the feeder circuit and trips the circuit breaker to disconnect the faulty feeder from the bus, which normally also supplies other feeders.
Modern protective relays are electronic, typically employing microprocessors, electronic displays, and data communications ports to exchange operating information with a central facility control system. The relays normally supply operating information, including load measurements, demand values, and circuit status to the control center. They require continuous power to perform these functions, as well as protection tasks during faults. The protective relays are commonly energized from a control power transformer connected to the monitored circuit between phases or from a phase to neutral, to step the power voltage down to a safe level suitable for the relay power supply input.
Under short circuit fault conditions generally, dangerous high currents will flow and the relay is expected to trip the breaker. However, the short circuit can also collapse the supply voltage causing loss of power to the protective relay, which will then fail to trip and isolate the fault. In general, multiple types of faults can cause the relay supply voltage to collapse below levels at which the relay can function, so users need a supply voltage source which is not solely dependent on the integrity of the monitored circuit. In weak-source applications, the supply voltage can collapse even with severe overloads, as well as for other faults. A common way of assuring a continued supply voltage source in large facilities is to provide a station backup battery supply. This solution, however, presents installation and maintenance cost concerns.
A second technique used to generate power for the protective relay is to employ auxiliary current transformers excited from the primary current transformers on the incoming power lines, that are used to supply the monitoring current input to the protective relay. This approach has the disadvantage of not generating sufficient power to energize the protective relay under very light current conditions. Also, the additional impedance presented by the auxiliary current transformers reduces the current available to the monitoring circuits which can also adversely affect the operating functions of the relay under light current conditions.
Accordingly, it is an object of this invention to provide an improved, reliable backup power source that does not require increased installation or maintenance costs. It is a further object to provide such a power source that maintains a sufficient input under light current conditions while not taxing the primary current transformers in a manner that would adversely affect the monitoring input to the protective relay.